Killing an enemy champion rewards a player with an amount of experience determined by the level of the champion slain. The experience gained is equal to 75% of the total experience required for a champion to go from the slain champion’s current level to the next level up. In other words, if a level 7 champion is slain, the total experience gained is 75% of the total experience required for a champion to go from level 7 to level 8, or 862.5 experience points. The experience reward will be increased if there is a positive level difference between champions (e.g. a level 5 champion killing a level 7 champion) and decreased if the level difference is negative.

EDIT:
Additionally, you have to factor in +exp masteries and runes, if you have either of them or both.

In addition, when more than 1 champion is sharing experience, 10% extra experience is split among all champions.